Scientific Forecaster Discussion
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Area forecast discussion...updated
National Weather Service Twin Cities/Chanhassen Minnesota
1116 PM CST Wed Nov 14 2018
Short term...(this evening through thursday)
issued at 241 PM CST Wed Nov 14 2018
Main concerns in the short term, temperature trends and timing and
chance of precipitation moving into the northern County Warning Area by late
Latest infrared and visible satellite showing some high clouds streaming
east/southeast along the northern tier of states associated with the
more zonal jet streak moving through the area. These clouds will
continue to sag southeast and should thin overnight. A breezy south
to southwest wind should help keep overall low temepratures on the
mild side, with readings generally for the mid 20s to lower 30s.
The next short wave trough embedded in the flow is now moving
through southern Alberta. It should advance to the east and across
the Dakotas by Thursday morning. A cold front associated with the
trough will move into the northwest County Warning Area early Thursday afternoon and
spread east to eastern Minnesota by 00z Friday. Clouds will thicken and
lower and will bring at least a small chance of light rain along the
far northern County Warning Area later Thursday afternoon. Highs Thursday afternoon
will be dependent on overall clouds cover with readings topping 50
again possible to the southwest before the front moves through.
Readings in the 40s will be common elsewhere.
Long term...(thursday night through wednesday)
issued at 241 PM CST Wed Nov 14 2018
Biggest concern in the long term is a light snow event expected to
impact southwest and south central Minnesota Friday afternoon and evening.
Otherwise, the period looks to be mainly dry with a bit of a roller
coaster for temperatures.
Thursday and Friday evenings will both feature fropas. The first
will bring back more of a Continental polar airmass, while fridays
front will signal the return of air originating from the Arctic. One
change we have seen with the 14.12 models is the Thursday front has
a bit more moisture to work with, with most models showing some
light precip working in behind the front across central Minnesota into
western WI. Precip looks very light, but there are indications in
the forecast soundings of there being potential for mainly
supercooled clouds. However, the supercooled cloud layers look
relatively shallow and occur within a cold advective regime, neither
of which is overly supportive of developing drizzle, so kept a
freezing rain/drizzle mention out of the forecast for Thursday
evening, but this will have to be monitored.
Friday, the Arctic front will come through during the day. A cross
section behind the front Friday afternoon/evening shows a rather
deep sloped frontal boundary coming through between about h85 and
h5. Though there are differences in quantitative precipitation forecast amounts and placement in the
models, everything shows precip impacting west central/southwest
into south central Minnesota between 18z Friday and 6z Saturday. Placement
for the core of the band in the 14.12 guidance was spread between
the Minnesota river on the north end (nam) to a Sioux Falls to Ames line on
the south end (canadian/ecmwf). A small enough spread where we felt
confident enough to boost blended pops up to the categorical level
for southwest/south central Minnesota. Beside qpf, snow amounts will also
be dependent on p-types. This region looks to be in the mid/upper
30s at precip onset, though it should cool quickly with the start of
precip, with only a brief period of rain or a rain/snow mix expected
before changing to all snow. Current forecast has a strip of 2-3" of
snow from about Canby down to St. James. This is a pretty quick
hitter, so even if this over achieves, we're only talking 4" type
Biggest change we've seen beyond Friday is the eastern trough is
expected to be a bit stronger and more sluggish to leave. There are
two implications here. One, it will be a slower warmup from our
coldest day on Saturday, with highs remaining cooler than normal
through Tuesday. In addition, we will be in the preferred clipper
zone in the northwest flow Monday into Tuesday and we are starting
to see some indications of another light snow event being possible
Monday afternoon/night. By Wednesday and Thanksgiving, the eastern
ridge will move off far enough east to allow for an upper ridge to
build into the plains, resulting in a couple of days similar to what
we are seeing today and Thursday. Also for the end of the week, the
European model (ecmwf) has started trending more toward the GFS/Canadian with a
weaker trough coming out of the southwest, keeping our area mostly
dry as our split flow regime continues.
Aviation...(for the 06z tafs through 06z Thursday night)
issued at 1116 PM CST Wed Nov 14 2018
VFR conditions tonight with lowering cigs late Thursday afternoon.
Snow showers are possible Thursday evening along with MVFR
cigs/visibilities. Winds south tonight, gradually veering
northwest by the end of the period.
Kmsp...no additional concerns.
/Outlook for kmsp/
Fri...MVFR cigs early. Chc -sn. Wind northwest 5-10 kts.
Sat...MVFR possible. Winds northwest 5-10 kts.
Sun...VFR. Winds SW 10 kts.